This invention relates to a positively working single-layer organic photoreceptor which can be mounted on high-resolution electrophotographic equipment of liquid (or wet) development system.
Electrophotographic image-forming apparatus have been widely spreading chiefly in offices as printers, copiers, facsimiles, etc. for their capability of high speed processing and high image quality.
Broadening of application of the electrophotographic image-forming techniques particularly in the fields of instruments having color output capability and light-duty printers has been of recent interest. Accordingly, electrophotographic image-forming technology realizing high resolution, high gradient and high speed has been being actively researched and developed, and market development of image-forming apparatus reflecting the results of the researches and developments is being undertaken.
Coping with such a market trend, improvements along the researches and developments have been added to a photoreceptor, which may be said to be the heart of this type of image-forming apparatus, particularly an organic photoreceptor.
This kind of organic photoreceptors are roughly divided into layered or dual-layer ones comprising a charge generating layer (CGL) having a charge generation function and a charge transport layer (CTL) having a charge transport function and single-layer ones having a photosensitive layer having both a charge generation function and a charge transport function.
The former type of organic photoreceptors comprise a cylindrical conductive substrate made of aluminum, etc. on which a CGL and a CTL are stacked in this order. Their application is generally limited to negatively charging image-forming apparatus in nature of the organic material. The latter type construction comprises a cylindrical conductive substrate of aluminum, etc. on which a single-ply photosensitive layer is provided and is mainly applied to positively charging image-forming apparatus which, in principle, easily provide high resolution.
When a semiconductor laser having a wavelength of 700 to 800 nm or a light-emitting diode having a wavelength of 600 to 700 nm is used as an exposure light source, both the single-layer and the layered photoreceptors often contain such photoconductive materials as phthalocyanine compounds, particularly metal-free phthalocyanine pigments and titanyl phthalocyanine pigments as a charge generating material (CGM).
Layered photoreceptors of separate function type in which a CTL is superposed on a CGL have been widely put into practical use for their electrophotographic characteristics and durability. However, because the CTL used in this type of photoreceptors chiefly contains a hole transport material (HTM) by material limitation, most of these photoreceptors adopt a negative charging layer configuration fit for negative charging electrophotographic processing.
Compared with positive corona discharge, negative corona discharge used in negative charging image-forming apparatus is less stable and generates more ozone, the powerful oxidizing action of which gives adverse influences to the photoreceptor and the working environment.
While a positive charging system generating less ozone is an effective choice to solve these problems, a positive charging organic photoreceptor required for this system is generally less sensitive than negative charging organic photoreceptors. Therefore, a positive charging organic photoreceptor with high sensitivity has been sought for. Because a positive charging photoreceptor has the charge generating function predominantly in the vicinity of its surface, the distance of electrons"" movement to the photoreceptor surface necessary to form an electrostatic latent image is shorter than that in a negative charging photoreceptor. As a result, a positive charging photoreceptor is characterized by high resolution.
Various layer configurations have been proposed for a photosensitive layer of a positive charging photoreceptor, including a layered type having a CGL on a hole transport layer (HTL) and a single-layer type having a layer containing both a charge generating material (CGM) and a charge transport material (CTM). Positive charging layered type photoreceptors are behind negative charging layered type photoreceptors in practical application because they have a thin CGL on their surface and therefore involve the problem of poor durability. Positive charging single-layer photoreceptors tend to be inferior to negative charging layered photoreceptors in electric characteristics such as sensitivity as stated above. This problem is due to because none of the available electron transport materials (ETMs) is equal or superior to available hole transport materials (HTMs) in mobility.
In recent years, a large number of ETMs and electrophotographic photoreceptors using them which deserve attention have been proposed or reported in JP-A-1-206349, JP-A-4-360148, Denshi Shashin Gakkaishi, vol. 30 (1991), pp. 266-273, JP-A-3-290666, JP-A-5-92936, Proceedings of Pan-Pacific Imaging Conference/Japan Hardcopy ""98, Jul. 15-17th, 1988, JA Hall, Tokyo, Japan, pp. 207-210, JP-A-9-151157, Proceedings of Japan Hardcopy ""97, Jul. 9-11th, 1998, JA Hall, Tokyo, Japan, pp. 21-24, JP-A-5-279582, JP-A-7-179775, Proceedings of Japan Hardcopy ""92, July 6-8th, JA Hall, Tokyo Japan, pp. 173-176, and JP-A-10-73937.
Further, single-layer photoreceptors having a combination of the HTM and the ETM disclosed in JP-A-5-150481, JP-A-6-130688, JP-A-9-281728, JP-A-9-281729, and JP-A-10-239874 have been noted for their high sensitivity, and part of them have been put to practical use.
On the other hand, electrophotographic apparatus are classified according to the development system into those for dry development and those for wet (liquid) development. The wet development system uses a liquid developer comprising charged toner particles dispersed in a dielectric medium, usually a petroleum isoparaffinic solvent such as Isopar (available from Exxon Chemicals). A photoreceptor having thereon an electrostatic latent image formed by exposure processing is dipped in or brought into contact with the liquid developer. Whereupon, the toner is electrophoretically adhered to the latent image in an electric field produced between the photoreceptor and a separately provided development electrode thereby to visualize the latent image. The toner used in the liquid development system has a smaller particle size (0.1 to 1 xcexcm) than that used in the dry development system (3 to 10 xcexcm) and a larger total surface area and thereby gains greater toner charge. As a result, toner image disturbances hardly occur, and the edge effect is reduced. Thus, the liquid development system is characterized by providing images of high resolution and good gradation reproduction and is particularly suitable for full color image formation.
Since the photoreceptor is dipped in a petroleum paraffinic solvent as mentioned above, the photosensitive layer must be resistant to this solvent. Because organic photoreceptors generally used in dry development are not sufficiently resistant to the solvent of this kind, they would undergo not only considerable deterioration in appearance but elution of the CTM dispersed therein into the solvent, resulting in serious reduction of electrical and optical characteristics. Therefore, inorganic photoconductive substances such as selenium and amorphous silicon have been of frequent use to make a solvent-resistant photoreceptor for use in the liquid development system.
With the recent development of organic photoreceptors, a solvent-resistant photoreceptor comprising a poly(2,6-dimethoxyanthracene-9,10-diol alkanedicarboxylic acid ester) resin has been proposed in JP-A-4-358157. The proposed photoreceptor is a negative charging layered photoreceptor having a solvent-resistant overcoat. The problem of harmful ozone generation due to negative corona discharge is still involved. It has therefore been keenly demanded to develop a positive charging single-layer photoreceptor that can be used in liquid development.
JP-A-2000-214610 discloses a positive charging single-layer photoreceptor format for wet development which has no overcoat and yet exhibits high resistance to Isopar, a main carrier medium used in wet development, involves little elution of the CTM, and exhibits high sensitivity for practical use. According to the present inventors"" investigation, however, the proposed photoreceptor cannot be seen as satisfactory in terms of sensitivity enough to meet the market demand, still leaving room for further improvement.
JP-A-2000-63456 discloses a positive charging single-layer photoreceptor for wet development which is free from deterioration by solvent and elution of the CTM into solvent, wherein a copolymer of a block having a chemical structure with a charge transport function and a block having a chemical structure as a binder resin is used. However, the sensitivity of the proposed single-layer photoreceptor cannot be necessarily seen as sufficiently high as to fulfill the market demand.
JP-A-5-230202 proposes a polycarbonate copolymer and an electrophotographic photoreceptor comprising the same. The polycarbonate copolymer comprises a diarylene vinylene arylene skeleton as a repeating unit and has a charge transport function. The disclosure refers to use of the polycarbonate copolymer in a single-layer photoconductive layer. However, the object of the invention is to improve wear resistance and electrophotographic characteristics, and to solve the problems of photoreceptors for wet development is not necessarily aimed at. There is found no reference still less suggestion as to durability against Isopar, a main carrier medium for wet development. It is unclear from the disclosure whether or not a photoreceptor having the single-layer photoconductive layer reaches high sensitivity in wet development.
JP-A-9-127713 discloses, in Table 1, a distyrylamine-polycarbonate copolymer resin comprising a distyrylamine skeleton (serving as an HTM) and a polycarbonate unit bonded to the para-position of a phenylene group bonded to the tertiary amino group of the distyrylamine skeleton. Although the disclosure suggests that the resin is applicable to a single-layer photoreceptor shown in FIG. 2, it lacks experimental confirmation that the resin is effective in a single-layer photoreceptor as well in a wet development system, seeing that all the photoreceptors prepared in Examples 1 to 16 are layered photoreceptors.
An object of the present invention is to provide a positive charging single-layer organic photoreceptor which has high resistance to an isoparaffinic solvent, a petroleum solvent used in liquid development, undergoes little elution of a CTM into the solvent, and exhibits sufficiently high sensitivity for practical use.
Other objects and effects of the invention will be apparent from the following description.
The objects of the present invention have been achieved by providing:
(1) A positive charging single-layer organic photoreceptor for liquid development which comprises a conductive substrate having thereon a photosensitive layer containing a CGM, an ETM, and an organic binder resin comprising a polymer represented by formula (I): 
wherein R1 and R2 each represent a substituted or unsubstituted alkyl group or a substituted or unsubstituted aryl group, or R1 and R2 may form a cycloalkyl group in combination; R3, R4, R5, R6, R7, R8, R9, R10 and R11 each represent a hydrogen atom, a halogen atom, an aryl group or an alkyl group; and m and n each represent a molar percentage giving a total of 70 to 95 mol %.
The present invention also provides the following preferred embodiments.
(2) The positive charging single-layer organic photoreceptor for liquid development according to item (1) above, wherein said photosensitive layer contains metal-free phthalocyanine.
(3) The positive charging single-layer organic photoreceptor for liquid development according to item (1) above, wherein said photosensitive layer contains titanyl phthalocyanine.
(4) The positive charging single-layer organic photoreceptor for liquid development according to any one of items (1) to (3) above, wherein said photosensitive layer contains a hole transport material.
(5) The positive charging single-layer organic photoreceptor for liquid development according to item (4) above, wherein the total content of said hole transport material and the component functioning for hole transport in said polymer represented by formula (I) is larger than the content of said electron transport material.
(6) The positive charging single-layer organic photoreceptor for liquid development according to any one of items (1) to (5) above, wherein said polymer represented by formula (I) is represented by formula (II), wherein m and n give a total of 80 to 90 mol %. 
(7) The positive charging single-layer organic photoreceptor for liquid development according to any one of items (1) to (6) above, wherein the content of said polymer in said photosensitive layer is 70% by weight or more.